Such are the principal modifications which normally take place during the development and growth of the fcetai portion of the pla centa. The changes occurring in the maternal portion, or that which is supplied by the de cidua, are not less remarkable.
Maternal portion.— Four principal stages may be observed in the formation of this por tion of the placenta.
The first stage is that in which the decidua constitutes a perfectly spherical chamber t surrounding the ovum, but having as yet no structural connection with it (fig. 486.). This is the condition of the ovum in the early part of the first month of gestation.
The second stage is marked by the com mencing attachment of the villi all round to the inner surface of the containing chamber, so that now the ovum becomes fixed, and can no longer be turned out, except by breaking off the villi, or drawing out their ends front the little pits, or anfractuosities, already de scribed in the walls of the decidua, in which they have become embedded. At this period (latter half of the first month), the decidua forming the walls of this chamber is suffi ciently firm to adniit of dissection, and already there may be traced, upon its inner surface, orifices communicating with canals in the de cidua that lead into the uterine sinuses. The maternal blood already flows freely into the fcetal chamber, and, after passing everywhere among the villi, is returned into the uterine veins. Thus a temporary placenta is formed, which, as in Pachydermata, Cetacea, &c., en tirely surrounds the ovum (fig. 487.).
The third stage is the most important. It marks the transition from the temporary to the permanent form of the placenta. Coinci dently with the increased development of the villi on one side of the chorion, and their cor responding arrest of growth on the opposite surface, there occurs an increase of the space between the decidua and the ovum on one side, and a corresponding decrease of it upon the opposite side. The increase is always on the side next the uterus, where the villi are most abundant, and the decrease upon the opposite surface, where they are fewest. And this change continues progressively, until, upon the bald side of the ovum, the decidua reflexa and the chorion come into so close contact that the interspace is obliterated, and the blood, which formerly flowed freely among the villi, is now no longer admitted to this part of the circumference of the ovum ; while, upon the side which is directed towards the uterus, a large space is left which now takes the form of a rneniscus. In order more effec
tually to confine the blood within this limited space, an increased development of decidual cells now takes place, which pass off from the uterine walls, and attach themselves to the chorion all round the circumference of this space, and thus is formed the margin of the permanent placenta. During all this time, the ovum, by its growth, has been gradually rais ing the decidua above and around it, just as the common integument becomes raised dur ing the formation of a subcutaneous abscess; while in proportion as the base of the chamber becomes extended by the gradual retiring, from the centre, of the line of reflexion of the de cidua, like waves receding from a central point, so, at the same time, an increasing surface is produced by the expansion of the uterus itself; and the layer of decidua here formed, com monly termed the decidua serotina, is sitnply the mucous membrane reproduced to supply the place of that which had been consumed or pushed °Win forming the decidua reflexa.
The fourth or final stage consists in the partitioning of the permanent placenta into smaller portions or lobes by the extension of the layer of decidua (serotina) which lies op posite to the developed villi inwards at vari ous points towards the chorion. In this way are constructed the disseviments already de scribed as bounding the several lobes or co tyledons. This partitioning of the placenta commences about the fourth month.
Thus, during these several stages in the formation of the placenta, two processes may be said to be concurrently carried on which tend in opposite directions — a process of positive enlargement and growth combined with one of relative retrogression or limita tion. For while the bulk of the placenta is progressively increasing up to the completion of pregnancy, the relative amount of surface of the ovum appropriated to it is, on the other hand, diminished. The entire surface of the chorion being, in the first stages of develop ment, employed as a placenta ; while in the latter half of gestation, one third of it suffices for that purpose.